Rotational molding, more commonly known as rotomolding, is widely used for molding hollow articles, and can be used to mold very large containers and tanks of more than 2000 gallons. To rotomold a part, a powdered resin is charged inside a mold shell, which is then rotated on two axes and heated inside an oven to cause the resin to adhere to the inside of the mold. After sufficient heating time, the mold is moved to a cooling chamber, and after cooling, the molded part is removed to begin another molding cycle. A more detailed discussion of rotomolding may be found in Modern Plastics Encyclopedia 1990, pages 317-318.
Rotational molding primarily uses polyolefin resins, with thermoplastic polymers of ethylene being principally used. Key properties for rotationally molded parts include appearance, and especially in the case of containers, resistance to puncture or rupture. It is essential for the resin, which is generally used in a powdered form of 35 mesh, to have sufficient melt flow to capture the texture and detail of the mold, and also to adhere quickly to the heated mold to reduce cycle time. A difficulty is that as melt flow increases, resistance to puncture or rupture, as measured for instance by impact strength, decreases, making it necessary to compromise the impact strength to obtain a good appearance on the outside of the molded part.
By comparison, in compression-molding, sufficient impact strength may be obtained to prevent puncture or rupture. However, rotomolding does not permit the application of pressure available in compression molding. Moreover, in compression molding, fractional melt index resins which have a higher molecular weight and a higher impact strength, may be used.
A useful additive for seeding crystals is a nucleating agent. Use of a nucleating agent for controlling foam cell structure in a foamable polyethylene-based composition during rotational molding, is described in U.S. Pat. No. 5,366,675 to Needham. Typically, a nucleating agent remains solid and is usually finely powdered. Nucleating agents include calcium carbonate; zinc oxide; pigments such as carbon black and titanium dioxide; and silicates such as kaolins, mica, talc, aluminum silicate, magnesium silicate, and calcium silicate. However, it is known that nucleating agents generally produce stiffer resins and consequently lower impact strength.
A problem with containers formed by rotational molding of commercially available, polyolefin-based compositions, is the tendency of the container wall to puncture or otherwise rupture, upon impact by a sharp object, for instance, a fork of a forktruck. Accordingly, there is a need for a polyolefin-based composition of improved impact strength, suited for rotomolding applications, and yet that has sufficient melt flow to capture the texture and detail of the mold.